Why does Moonshadow react to Miss Whitelaw's stained glass window?

When Miss Whitlaw shows Windrider and his father her stained-glass window, Windrider is amazed that the window depicts a great dragon reigning terror on people. Moon Shadow immediately points this out to his father, noting that the depiction is incorrect. Windrider then explains that demons do not know the truth about dragons. In response, Moon Shadow decides that it should be his job to teach the kindly Miss Whitlaw the truth.